U.S. patent number 4,975,919 [Application Number 07/296,120] was granted by the patent office on 1990-12-04 for laser wavelength control apparatus.
This patent grant is currently assigned to Kabushiki Kaisha Komatsu Seisakusho. Invention is credited to Yoshiho Amada, Masahiko Kowaka, Osamu Wakabayashi.
United States Patent |
4,975,919 |
Amada , et al. |
December 4, 1990 |
Laser wavelength control apparatus
Abstract
According to present invention, the central wavelength of the
central wave of the output laser beam, and the sideband wave power
or central wave power of the output laser beam are detected, and
the wavelength selective characteristics of wavelength selective
elements disposed between a laser chamber and a rear mirror are
controlled such that the detected central wavelength falls within a
desired allowable range and that the detected power becomes minimum
or maximum.
Inventors: |
Amada; Yoshiho (Kanagawa,
JP), Wakabayashi; Osamu (Kanagawa, JP),
Kowaka; Masahiko (Kanagawa, JP) |
Assignee: |
Kabushiki Kaisha Komatsu
Seisakusho (Tokyo, JP)
|
Family
ID: |
13272049 |
Appl.
No.: |
07/296,120 |
Filed: |
November 17, 1988 |
PCT
Filed: |
March 18, 1988 |
PCT No.: |
PCT/JP88/00293 |
371
Date: |
November 17, 1988 |
102(e)
Date: |
November 17, 1988 |
PCT
Pub. No.: |
WO88/07276 |
PCT
Pub. Date: |
September 22, 1988 |
Foreign Application Priority Data
|
|
|
|
|
Mar 19, 1987 [JP] |
|
|
62-64923 |
|
Current U.S.
Class: |
372/33; 372/32;
372/99 |
Current CPC
Class: |
H01S
3/137 (20130101); H01S 3/225 (20130101) |
Current International
Class: |
H01S
3/13 (20060101); H01S 3/137 (20060101); H01S
3/14 (20060101); H01S 3/225 (20060101); H01S
003/00 () |
Field of
Search: |
;372/28,32,34,57,98,99,33 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
4150342 |
April 1979 |
Johnston, Jr. et al. |
4829536 |
May 1989 |
Kajiyama et al. |
|
Primary Examiner: Gonzalez; Frank
Attorney, Agent or Firm: Diller, Ramik & Wight
Claims
We claim:
1. A laser wavelength control apparatus, characterized by:
a rear mirror comprising a total reflection mirror;
a front mirror disposed opposite to the rear mirror, for permeating
a part of laser beam;
a laser chamber disposed between the rear mirror and the front
mirror, in which a laser material is filled;
at least two wavelength selective elements disposed at an optical
path of the laser beam between the rear mirror and the laser
chamber;
separating means for separating an output laser beam outputted from
the front mirror into main peak components and sideband wave
components;
central wavelength detecting means for detecting a central
wavelength of the main peak components of the output laser beam
separated by the separating means;
sideband wave power detecting means for detecting a power of the
sideband wave components of the output laser beam separated by the
separating means;
first control means for controlling wavelength selective
characteristics of the wavelength selective elements so that the
value of the central wavelength detected by the central wavelength
detecting means falls within a first desired allowable range;
and
second control means for controlling the wavelength selective
characteristics of the wavelength selective elements so that the
value of the sideband wave power detected by the sideband wave
power detecting means becomes minimum.
2. A laser wavelength control apparatus according to claim 1,
wherein the separating means includes a spectroscope, the central
wavelength detecting means includes:
a photodiode array for receiving the main peak components separated
by the spectroscope; and
means for detecting the central wavelength from a light-receiving
channel composed of the photodiode array receiving the main peak
components, and the sideband wave power detecting means
includes:
the photodiode array receiving the sideband wave components
separated by the spectroscope together with the main peak
components; and
means for detecting the sideband wave power from an output level of
the light-receiving channel of the sideband wave components.
3. A laser wavelength control apparatus according to claim 1,
wherein the separating means includes a spectroscope, the central
wavelength detecting means includes a position sensitive device for
receiving the main peak components separated by the spectroscope
and for detecting the central wavelength from the light-receiving
position, and the sideband wave power detecting means includes a
photodiode for receiving the sideband wave components separated by
the spectroscope and for detecting the sideband wave power from the
received light output.
4. A laser wavelength control apparatus according to claim 1,
wherein the separating means includes a spectroscope and mirror
means for taking out the sideband wave components from the light
separated by the spectroscope, the central wavelength detecting
means includes first light-receiving means for detecting the
central wavelength from the main peak components separated by the
spectroscope, and the sideband wave power detecting means includes
second-light receiving means for detecting the sideband wave power
from the sideband wave components taken out by the mirror
means.
5. A laser wavelength control apparatus according to claim 1,
wherein the separating means includes a spectroscope, the central
wavelength detecting means includes light-receiving means for
receiving the main peak components separated by the spectroscope,
the sideband wave power detecting means includes:
first light-receiving means for receiving the main peak components
separated by the spectroscope;
second light-receiving means for receiving the main peak components
and the sideband wave components separated by the spectroscope;
and
means for detecting the sideband wave power from the difference
between the received light output of the second light-receiving
means and the received light output of the first light-receiving
means.
6. A laser wavelength control apparatus according to claim 1,
wherein the at least two wavelength selective elements comprise a
first etalon having a large free spectral range and a second etalon
having a small free spectral range, and wherein the first control
means controls the wavelength selective characteristic of the
second etalon, and the second control means controls the wavelength
selective characteristic of the first etalon.
7. A laser wavelength control apparatus according to claim 1,
wherein the control of the wavelength selective characteristics of
the etalons by the first and second control means is effected by
changing at least one selected from the angle, temperature, gap
pressure and gap spacing of the etalons.
8. A laser wavelength control apparatus according to claim 1,
wherein the separating means includes a spectroscope for separating
the output laser beam into the main peak components and the
sideband wave components, the spectroscope separating a light of
higher order than second order.
9. A laser wavelength control apparatus, characterized by:
a rear mirror comprising a total reflection mirror;
a front mirror disposed opposite to the rear mirror, for permeating
a part of laser beam;
a laser chamber disposed between the rear mirror and the front
mirror, in which a laser material is filled;
two etalons disposed at an optical path between the rear mirror and
the laser chamber;
means for detecting a central wavelength and a central wavelength
power of an output laser beam outputted from the front mirror;
first control means for controlling a wavelength selective
characteristic of one of the two etalons having the small free
spectral range so that the central wavelength detected by the
detecting means falls within a desired allowable range; and
second control means for controlling a wavelength selective
characteristic of the other one of the two etalons so that the
central wavelength power detected by the detecting means becomes
maximum.
10. A laser wavelength control apparatus according to claim 9,
wherein the detecting means includes a spectroscope for separating
the main peak components from the output laser beam, and a
one-dimensional light-receiving element for receiving the main peak
components separated by the spectroscope and for detecting the
central wavelength and the central wavelength power of the output
laser beam.
11. A laser wavelength control apparatus according to claim 9,
wherein the control of the wavelength selective characteristics of
the etalons by the first and second control means is effected by
changing at least one selected from the angle, temperature, gap
pressure, and gap spacing of the etalons.
12. A laser wavelength control apparatus, characterized by:
a rear mirror comprising a total reflection mirror;
a front mirror disposed opposite to the rear mirror for permeating
a part of laser beam;
a laser chamber disposed between the rear mirror and the front
mirror, in which a laser material is filled;
at least two wavelength selective elements disposed at an optical
path between the rear mirror and the front mirror;
reference beam generating means for generating a reference beam
having a reference wavelength;
detecting means for receiving an output laser beam outputted from
the front mirror together with the reference beam and for detecting
a central wavelength of the output laser beam from a relative
wavelength difference relative to the reference beam; and
control means for controlling wavelength selective characteristics
of the wavelength selective elements so that the central wavelength
detected by the detecting means falls within a desired allowable
range.
13. A laser wavelength control apparatus according to claim 12,
wherein the reference beam generating means comprises a mercury
lamp.
14. A laser wavelength control apparatus according to claim 12,
wherein the reference beam generating means comprises an argon ion
laser.
15. A laser wavelength control apparatus according to claim 12,
wherein the reference beam generating means comprises an argon ion
laser and means for doubling the frequency of the output beam of
the argon ion laser.
16. A laser wavelength control apparatus according to claim 12,
wherein the detecting means comprises a spectroscope for detecting
the output laser beam and the reference beam simultaneously.
17. A laser wavelength control apparatus according to claim 12,
wherein the detecting means comprises a monitor etalon for
detecting the output laser beam and the reference beam
simultaneously.
Description
TECHNICAL FIELD
The present invention relates to laser wavelength control apparatus
and, more particularly, to such apparatus for a narrow-band
oscillation excimer laser suitable for use as a light source of a
reduction projection aligner.
BACKGROUND ART
The use of an excimer laser as a light source of a reduction
projection aligner for semiconductor device production attracts
public attention. This is because many excellent advantages are
expected: i.e., there is the probability that the limit of the
resolution will be enhanced to less than 0.5 um since the
wavelength of the excimer laser is short (the wavelength of a KrF
laser is about 248.4 nm); the depth of the focus is great compared
to the g-and i-lines from a mercury lamp used conventionally with
the same resolution: the numerical aperture (NA) of a lens may be
reduced; a large exposure area is available, high power is
available, etc.
However, there are two big problems to be solved when the excimer
laser is used as the light source of a reduction projection
aligner.
One is that since the wavelength of the excimer laser is short,
namely, 248.35 nm, the materials transparent to this wavelength are
only quartz, CaF.sub.2 and MgF.sub.2, and only quartz can be used
as the lens material among these named materials from a standpoint
of uniformness and working accuracy. Therefore, it is impossible to
design a reduction projection lens with corrected chromatic
aberration. Thus, it is necessary to narrow the bandwidth of the
excimer laser to such an extent that the chromatic aberration is
negligible.
The other problem is how to suppress a speckle pattern produced by
narrowing the excimer laser band and how to suppress a reduction of
power produced by narrowing the band.
There is a technique of narrowing the excimer laser band called an
injection locking system. This system includes wavelength selective
elements (an etalon, a diffraction grating, a prism, etc.,)
disposed within a cavity of an oscillator stage. The system is
oscillated in a single mode by limiting a spatial mode using a
pinhole, the laser beam is injection synchronized by an amplifier
stage. Thus, the output beam is high in coherency. If this output
beam is used as a light source for the reduction aligner, a speckle
pattern will be generated. Generally, the generation of a speckle
pattern is considered to be dependent on the number of spatial
transverse modes contained in the laser beam. It is known that if
the number of spatial transverse modes contained in the laser beam
decreases, a speckle pattern becomes likely to be generated whereas
if the number of spatial transverse modes increases, the speckle
pattern becomes less likely to be generated. The injection locking
system is a technique of narrowing the band essentially by greatly
reducing the number of spatial transverse modes. However, the
generation of a speckle pattern is a big problem, so that the
injection locking system cannot be employed in a reduction
projection aligner.
A promissing technique of narrowing the excimer laser band uses
etalons which is a wavelength selective element. As a conventional
technique using etalons, AT & T Bell Laboratories has proposed
a technique of narrowing the excimer laser band by disposing
etalons between a front mirror and a laser chamber of the excimer
laser. However, there is the problem that in this system the
spectral profile cannot be narrowed so greatly, and that power loss
is large due to insertion of the etalon, and the drawback that the
number of spatial transverse modes cannot be increased so
greatly.
Inventors have succeeded in obtaining a laser beam having an output
of about 50 mJ per pulse by uniformly narrowing the spectral width
to within about 0.003 nm in full width at the half maximum over
whole beam size of 20.times.10 mm in which an etalon having a large
effective diameter (of about dozen millimeters) is disposed between
the rear mirror and laser chamber of the excimer laser. By
employing the structure in which the etalon is disposed between the
rear mirror and the laser chamber of the excimer laser, essential
problems required for a light source for a reduction projection
aligner including narrowing the laser band, ensuring the number of
spatial transverse modes, and reduction of the power loss due to
insertion of the etalon have been solved.
Although the structure in which the etalon is disposed between the
rear mirror and laser chamber of the excimer laser has excellent
advantages such as narrowing the band, ensuring the number of
spatial transverse modes, and reduction of the power loss, power
transmitting through the etalon is very high, so that physical
changes fluctuations in the etalon temperature will occur.
Therefore, there is the problem that the central wavelength of the
oscillated output laser beam may fluctuate and the power may be
greatly reduced. This tendency is especially remarkable in the use
of two or more etalons with different free spectral range for
narrowing the band.
It is an object of the present invention to provide in a laser
having a structure in which a wavelength selective elements are
disposed between the rear mirror and the laser chamber of the laser
a laser wavelength control apparatus in which the central
wavelength of the output laser beam is locked with high accuracy to
reduce fluctuations of the laser power to thereby provide a
stabilized output.
DISCLOSURE OF THE INVENTION
In order to achieve the above object, according to the present
invention, the central wavelength of the central wave and the power
of the sideband waves of the the output laser beam are detected,
and the wavelength selective characteristic of wavelength selective
elements disposed between the laser chamber and the rear mirror of
the laser is controlled such that the detected central wavelength
falls in a desired allowable range and that the power of the
detected sideband waves falls in a desired allowable range (a
minimum value).
Thus, according to the present invention, there is provided a laser
wavelength control apparatus in which at least two wavelength
selective elements are disposed between a laser chamber and a rear
mirror of the laser, and the central wavelength and spectral
profile of the output laser beam are controlled by controlling the
respective wavelength selective characteristics of the wavelength
selective elements, characterized by means for detecting the
central wavelength of the central wave of the output laser beam and
detecting the power of the sideband waves of the output laser beam;
first control means for controlling the wavelength selective
characteristics of the wavelength selective elements such that the
value of the central wavelength detected by the detecting means
falls within a first desired allowable range; and second control
means for controlling the wavelength selective characteristics of
the wavelength selective elements such that the value of the
sideband wave power detected by the detecting means falls within a
second allowable range.
According to the present invention, the central wavelength and
power of the central wave of the output laser beam are detected,
and the wavelength selective characteristic of wavelength selective
elements or etalons disposed between the laser chamber and the rear
mirror of the laser is controlled such that the detected central
wavelength falls in a desired allowable range and that the detected
power becomes a maximum value.
Thus, according to the present invention, there is also provided a
laser wavelength control apparatus in which two etalons with
different free spectral range are disposed between a laser chamber
and a rear mirror of the laser, and the wavelength selective
characteristics of the etalons are controlled to control the
central wavelength and spectral profile of the output laser beam,
characterized by means for detecting the central wavelength and
central wavelength power of the output laser beam; first control
means for controlling the wavelength selective characteristic of at
least one, with smaller free spectral range, of the etalons such
that the central wavelength detected by the detecting means falls
within a desired allowable range; and second control means for
controlling the wavelength selective characteristic of at least
one, with larger free spectral range, of the etalons such that the
central wavelength power detected by the detecting means becomes
maximum.
In the present invention, reference generating means which
generates a reference beam having a predetermined wavelength is
used in the detection of the central wavelength.
Thus, according to the present invention, there is provided a laser
wavelength control apparatus in which at least one wavelength
selective element is disposed between a laser chamber and a rear
mirror of the laser, and the central wavelength and spectral
profile of the output laser beam are controlled by controlling the
respective wavelength selective characteristics of these wavelength
selective elements, characterized by reference beam generating
means for generating a reference beam having a reference
wavelength, detecting means for receiving the output laser beam
together with the reference beam and for detecting the central
wavelength of the output laser beam from a relative wavelength
difference relative to the reference beam; and control means for
controlling the wavelength selective characteristics of the
wavelength selective elements such that the central wavelength
detected by the detecting means falls within a desired allowable
range.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing one embodiment of the present
invention;
FIGS. 2 are schematics showing an illustrative structure of the
wavelength detector;
FIGS. 3 are spectral diagrams showing the state in which etalons #1
and #2 are superposed over each other;
FIGS. 4 are flowcharts showing illustrative control by a
controller;
FIGS. 5 and 6 illustrate the relationship between etalon angle and
oscillating wavelength;
FIG. 7 is a flowchart showing another illustrative control of the
controller; and
FIG. 8 is a block diagram showing another embodiment using
reference light.
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a block diagram showing one embodiment of the present
invention. A laser device 1 has an oscillating circuit in which a
rear mirror 2, a front mirror 3 and a laser chamber 4 filled with a
laser gas are disposed. Two etalons: i.e., a first etalon #1 and a
second etalon #2, are disposed as wavelength selective elements
between the rear mirror 2 and the laser chamber 4 such that the
oscillated laser beam is output through a half mirror 6 out of the
device. The laser beam reflected by the half mirror 6 is entered
via a lens 7 into an optical fiber 8 and further input to a
spectroscope 10 via a lens 9 provided at the other end of the
optical fiber.
The spectroscope 10 has a well-known Czerny-Turner type structure
in which incident light is transmitted through a mirror 100, a
concave mirror 101, a diffraction grating 102, and a concave mirror
103 in this order. Thus, the central wave (main peak) component and
sideband wave (side peak) components are extracted separately,
which are then entered into a wavelength detector 105. This
detector 105 detects the central wavelength of the output laser
beam from the incident central wave component and the power of the
sideband wave from the sideband components.
The output signal from the wavelength detector 105 is input to a
controller 11, which controls, for example, the respective angles
of two etalons #1 and #2 on the basis of the central wavelength and
sideband wave power detected by the wavelength detector 105.
Arrangement may be such that other controlled elements such as
etalon temperature, gap pressure, and gap spacing can be
changed.
FIGS. 2(a)-(d) shows an illustrative structure of the wavelength
detector 105. The detector shown in FIG. 2(a) includes a
one-dimensional diode array 12, which in turn includes a plurality
of photodiodes arranged in a line, as well known. The
photodetection position is detected by a photodiode which detects
laser beam, namely, a photodetection element (photodetection
channel) which produces an output and the power is detected by the
level of the output from the photodetection element. Therefore,
with the structure of FIG. 2(a), the central wavelength of the
output laser beam from the photodetection channel due to the
central wave components (shown by the dot-dashed line in FIG. 2(a))
is detected. The sideband wave power may be detected by the output
level of the photodetection channel due to the sideband wave
components (shown by the broken line in FIG. 2(a)).
FIG. 2(b) shows a wavelength detector which includes two
photodiodes 13 and 14, an one dimensional PSD (Position Sensitive
Device) 15. The photodiodes 13 and 14 are disposed at positions
where the sideband wave components are detected, and the PSD 15 is
disposed at a position where the central wave component is
detected. According to the structure of FIG. 2(b), the central
wavelength is detected from the output of the PSD 15, and the power
of the sideband waves is detected from the outputs of the
photodiodes 13 and 14.
FIGS. 2(c) and (d) each are a wavelength detector suitable for use
when the central wave component and the sideband wave components
occur close to each other. In FIG. 2(c), the wavelength detector
includes photodiodes 13 and 14, PSD 15 and two half mirrors 16 and
17. In the arrangement, the central wave component is detected by
the PSD 15, and the central wavelength is detected from the output
of the PSD 15. The sideband wave components are detected by the
photodiodes 13 and 14 via the half mirrors 16 and 17 and the power
of the sideband waves is obtained from the outputs of the
photodiodes 13 and 14.
The structure of FIG. 2(d) includes two PSDs 15 and 15' and a
single half mirror 17. In the arrangement, the central wave
component is detected by the PSD 15, and the central wavelength and
the central wave component power are detected from the PSD 15. The
central and sideband wave components reflected by the mirror 17 are
received by the PSD 15 from which the sum of the central and
sideband wave component powers is detected. Therefore, the
subtraction of the output of the PSD 15 from the output of the PSD
15' results in the detection of the sideband component power. The
etalons #1 and #2 disposed between the rear mirror 2 and the laser
chamber 4 are composed of an etalon with large free spectral range
and an etalon with small spectral range compared to the etalon #1,
respectively. In this case, the central wavelength of the output
laser beam is mainly determined by the controlled state of the
etalon #2 with small spectral range (the central wavelength does
not change greatly by controlling the etalon #1 with small spectral
range).
FIG. 3 shows that situation. FIG. 3(a) shows the state in which the
etalons #1 and #2 are superposed over each other (the selected
wavelengths passing through the etalons #1 and #2 coincide and the
central wavelength power is maximum). The broken line shows the
wavelength spectrum passing through the etalon #1 with large free
spectral range while the solid line shows the wavelength spectrum
passing through the etalon #2 with small free spectral range.
As shown in FIG. 3(b), if the wavelength passing through the etalon
#1 shifts and the etalon #1 deviates in position from the etalon
#2, the central wave component power decreases and the sideband
wave components appear. However, in this case, the central
wavelength does not change.
As shown in FIG. 3(c), if the etalon #1 further deviates from the
etalon #2, the central wave component power further decreases while
the sideband wave component power further increases.
As is obvious from FIG. 3, the central wavelength of the output
laser beam is determined by the controlled state of the etalon #2
with small spectral range. When the etalons #1 and #2 are
superposed, the sideband wave power is minimum while the central
wavelength power is maximum. To the contrary, if the etalons #1 and
#2 are not superposed, the sideband wave power is significant while
the central wavelength power is small.
In this embodiment, the central wavelength control is performed in
which the etalon #2 with small spectral range is controlled on the
basis of the central wavelength detected by the wavelength detector
105 to thereby lock the output laser beam to a desired central
wavelength, and superposing control is performed in which the
etalon #1 with large free spectral range is controlled so as to
minimize the sideband wave power detected by the wavelength
detector 105 to thereby cause the etalons #1 and #2 to be
superposed.
FIGS. 4(a) and (b) illustrate flowcharts in which the etalons #1
and #2 are controlled by the controller 11.
FIG. 4(a) shows the above mentioned central wavelength control. The
central wavelength is detected by the wavelength detector 105 (step
201). It is determined whether the detected central wavelength is
within an allowable range (step 202). If the central wavelength is
out of the allowable range, the angle of the etalon #2 with small
free spectral range is adjusted such that the central wavelength
falls within the allowable range.
FIG. 4(b) illustrates the above mentioned superposing control. The
peak value (power) of the sideband wave component is detected (step
204). If the peak value is out of the allowable range (step 205),
the angle of the etalon #1 with large free spectral range is
adjusted (step 206) such that the peak value falls within the
allowable range.
The central wavelength control shown in FIG. 4(a) and the
superposing control shown in FIG. 4(b) are repeated alternately or
simultaneously.
FIGS. 5 and 6 illustrate the state in which the etalons #1 and #2
are subjected to superposing control. Assume now that the angle of
the etalon #2 with small free spectral range is fixed as shown in
FIG. 6, and that the angle of the etalon #1 with large free
spectral range is in the state A. At this time, the central
wavelength passing through etalon #1 is on the lower wavelength
side of the central wavelength passing through the etalon #2, and
the etalons #1 and #2 are not superposed (FIG. 5(a)). Here, if the
angle of the etalon #1 is changed from the state A to the state B,
the central wavelength passing through the etalon #1 shifts to the
higher wavelength side, so that the etalons #1 and #2 are
superposed (FIG. 5(b)). If the angle of the etalon #1 is changed
from the state B to the state C, the central wavelength passing
through the etalon #1 shifts to further higher wavelength side, so
that the etalons #1 and #2 are not superposed (FIG. 5(c)).
Therefore, in this case, the angle of the etalon #1 may be
controlled so as to be in the state C in FIG. 6.
While in the above embodiment the superposing control of the
etalons #1 and #2 is performed by controlling the wavelength
passing through the etalon #1 such that the sideband wave power
falls within a predetermined allowable range, arrangement may
instead be such that the central wavelength power is detected and
that the wavelength passing through the etalon #1 is controlled in
such a manner that the central wavelength power becomes
maximum.
One example of the superposing control in this case is illustrated
in the flowchart of FIG. 7. Namely, the peak value (power) of the
central wavelength component is detected (step 304). If the peak
value is not maximum (step 305), the angle of the etalon #1 with
large free spectral range is adjusted (step 306) such that the peak
value becomes maximum.
FIG. 8 illustrates another embodiment using reference light.
In the particular embodiment, a reference beam generator 23 is
provided which generates a light beam having a reference wavelength
beam such as argon ion laser beam, a beam having twice the
frequency of the argon ion laser beam, or mercury lamp light. The
laser beam generated by the laser device 1 is entered into a
spectroscope 24 via half mirrors 21 and 22. Simultaneously, the
reference light beam from the reference beam generator 23 is
entered into the spectroscope 24 via the half mirror 22. The
spectrometer 24 detects the central wavelength of the laser beam,
using, as a reference, the reference light wavelength separated by
spectroscope 24. The controller 25 adjusts the angle of the etalon
20 on the basis of this detected value. Thus it is possible to
control the wavelength of the laser beam to a desired absolute
wavelength in a stabilized manner.
In that case, the reference beam may be entered periodically into
the spectroscope 24.
The laser wavelength control apparatus may be constructed using a
monitor etalon instead of the spectroscope 24 in the particular
embodiment.
The number of etalons provided between the rear mirror and the
laser chamber is not limited to two and may be three or more.
A laser wavelength control apparatus may similarly constructed
using other wavelength selective elements instead of etalons.
INDUSTRIAL APPLICABILITY
As described above, according to the present invention, a narrow
band output of stabilized power and central wavelength is provided.
For example, when a laser to which the present invention is applied
is used as a light source of a reduction projection aligner,
stabilized focal position, magnification and high resolution are
obtained. Furthermore, the exposure time becomes constant and
exposure quantity control becomes easy. A laser to which the
present invention is applied may be used in medical applications
(especially in ophthalmic applications).
* * * * *